At the present time photovoltaic or solar cells (SC) are classified into three generations, which are described below.
First-generation solar cells are silicon-based solar cells (Si—SC) that dominate the solar market (80 to 90%). Solar cells of this type are manufactured of mono-crystalline or polycrystalline silicon, and, in spite of high manufacturing cost (typically ranging from $3/W to $5/W which is much higher than is required for wide implementation), popularity of these SC results from their high efficiency, well developed processing, and practically unlimited availability of silicon.
Solar cells of the second generation are also known as thin-film solar cells (TF-SC). The cells of this type are less expensive, lighter in weight, and more attractive in appearance than solar cells of the first generation. However, they are less efficient than first-generation cells.
Third-generation cells contain a wide range of potential device and material innovations, including organic SC, nanomaterial-based cells, dye-synthesized SC (DSSC) and others.
Irrespective of a provision of later generations, interest in SC of the first generation remains very keen, and research in this direction continues. The high fabrication cost of the first-generation Si—SC results mainly from several high-temperature processes required to form emitters and selective emitters, doped base contact regions, passivation and anti-reflection layers, metal contacts, back-surface field (BSF) regions, which are required on front-side SC, and front-surface field (FSF) regions, which are required on backside SC, etc. The aforementioned set of functional SC regions represent the SC functional structure. Emitters and selective emitters, doped base contact regions, BSF and FSF regions generally require doping which is different from the substrate doping The present application applies specifically to the Si-based backside SC which will be further referred to as BS-SC.
BS-SC functional structure and fabrication of BS-SC are described e.g., in “High-efficiency back-contact back junctionsilicon solar cells research at Fraunhofer ISE” by F. Granek et al., Proceedings 23rd European Photovoltaic Solar Energy Conference, Valencia, Spain, pp. 991-995 (2008).
Generally the BS-SC fabrication requires high-temperature diffusion for all doped functional regions of the SC such as backside emitters and selective emitters, backside doped base contact regions and FSF (see e.g. U.S. Pat. No. 7,897,867 issued on Mar. 1, 2011 to W. P. Mulligan et al.) All diffusions are typically performed in high-temperature thermal diffusion furnaces, belt furnaces, and may require rapid thermal annealing (RTA) chambers. Diffusion and annealing processes are generally power-consuming and time-consuming, and equipment with which these processes are carried out generally requires periodic calibration, testing, and maintenance. Another source of complexity and cost increase in the manufacture of first-generation SC is patterning, a process that typically involves the use of several photolithography operations or other masking steps for forming selective emitters, base contact regions, metal electrodes, and other cell elements.
Attempts have been made to simplify fabrication of BS-SC, e.g., by reducing the number of masking, diffusion, and passivation steps, which are used in screen printing or jet printing with consecutive annealing of screen-printed layers. For example, conductive electrodes can be formed by the back-side screen-printing and annealing technique as described e.g. in U.S. Patent Application Publication No. 20090025786, published on Jan. 29, 2009, inventors: Ajeet Rohatgi, et al).
U.S. Pat. No. 7,897,867 issued on Mar. 1, 2011 to W. P. Mulligan et al, for example, described BS-SC fabrication where printing techniques are utilized in selectively forming masks for use in etching of silicon oxide and diffusing dopants and in forming metal contacts to the diffused regions. The patent also describes the process of forming self-aligned stack of metals for backside contacts. While the described method may be less expensive than a regular microelectronic circuit processing it sill requires several high-temperature furnace-based diffusions for forming backside emitters, backside doped base contact regions and FSF.
U.S. Pat. No. 7,928,015 issued on Apr. 19, 2011 to David K. Fork describes the SC fabrication in which wafer-based solar cells are efficiently produced by extruding a dopant bearing material (dopant ink) onto one or more predetermined surface areas of a semiconductor wafer, and then thermally treating the wafer to cause diffusion of dopant from the dopant ink into the wafer to form corresponding doped regions. A multi-plenum extrusion head is used to simultaneously extrude interdigitated dopant ink structures having two different dopant types (e.g., n-type dopant ink and p-type dopant ink) in a self-registered arrangement on the wafer surface. The described method provides simultaneous deposition on a silicon wafer of several pre-patterned doping sources, but the actual doped regions are consequently formed by a thermal diffusion as usual.
U.S. Pat. No. 7,820,475 issued on Oct. 26, 2010 to Dennis De Ceuster et al. describes the process with fewer diffusion steps in which the functional doped regions on the backside of the wafers (such as selective emitters and doped base contact regions) and FSF are formed and patterned by screen-printing and etching of several doping sources deposited on a substrate. In this method at least one high-temp diffusion is still required to form the BS-SC functional structure.
Another printing method of patterning of a SC functional structure is described in the paper titled “Inkjet printing for high efficiency selective emitter solar cell”, Proceedings of 23rd European Photovoltaic Solar Energy Conference, Valencia, Spain, pp. 1687-1690 (2008). It should be noted that in this and preceding reference a high-temp furnace-based diffusions remain necessary for forming doped functional regions of SC.
US Patent Application Publication No. 20100068848 (published on Mar. 18, 2010; inventors: Kuo, Ming-Chin, et al) describes a one-step diffusion method of fabricating light doping regions and heavily doped regions in the Si—SC.
Another efficient attempt to minimize the number of diffusion, passivation, and masking operations in solar cell fabrication is disclosed in U.S. Pat. No. 7,615,393 issued on Nov. 10, 2009 to Sunil Shah, et al. The method described in this patent provides a substrate that is doped with boron and includes a first substrate surface with a first surface region and a second surface region. A first set of nanoparticles, which includes a first dopant, is deposited on the first surface region. The substrate is heated in an inert ambient to a first temperature, whereby a first densified film is created, and then a first diffused region (that later serves as a selective emitter) is formed with the first diffusion depth in the substrate beneath the first surface region. The method also includes exposing the substrate to a diffusion gas that includes phosphorous at a second temperature for forming a phosphorosilicate glass (PSG) layer on the first substrate surface, and then a second diffused region with a second diffusion depth (that serves as a lower doped emitter) is formed in the substrate beneath the second surface region wherein the first diffused region is proximate to the second diffused region. The method further includes exposing the substrate to an oxidizing gas at a third temperature, wherein an SiO2 layer is formed between the PSG layer and the substrate surface, wherein the first diffusion depth is substantially greater than the second diffusion depth (i.e. the selective emitter regions are deeper and heavier doped than the lower doped emitter region). Thus, multidoped junctions are formed on a substrate essentially without photolithography.
While this method represents an interesting advance toward simplification of solar cell manufacturing and can be applied to BS-SC fabrication, it still requires at least one complex thermal diffusion process (step that includes using a dopant gas). Also, diffusion of phosphorus onto the front surface is conducted simultaneously with diffusion of aluminum onto the back side, which may cause uncontrolled doping on the back-side doped regions. Furthermore, this method requires alignment of the metal electrodes to the selective emitters and doped base contact regions, which is not done automatically and which may involve additional steps.
U.S. Patent Application Publication No. 20100012185 (published on Jan. 21, 2010; inventors: Christian Schmid, et al) and U.S. Pat. No. 6,262,359 issued on Jul. 17, 2001 to Daniel Meier, et al, describe a process wherein aluminum or aluminum-containing paste is deposited on the back side of a solar cell and is annealed to create a back-surface field (BSF) region without performing a thermal diffusion step. Also in the paper titled “Towards 20% efficient N-type silicon solar cells with screen-printed aluminum-alloyed rear emitter”, Proceedings of 23rd European Photovoltaic Solar Energy Conference, Valencia, Spain, pp. 982-987 (2008) the process of forming backside Al-doped emitter is described, which includes using screen-printing of Al-based paste and annealing in the belt furnace. The described method also includes a photolithography and a thermal diffusion steps performed on the front surface.
U.S. Pat. No. 6,429,037 issued on Aug. 6, 2002 to Stuart R. Wenham, et al, discloses a method for forming selective emitters without recourse to a conventional diffusion step generally required for the formation of heavily doped regions of selective emitters. This is achieved by means of laser-assisted local heating of a dopant source that also serves as a passivation layer and mask for consequent metallization. The method also allows formation of self-aligned contacts on selective emitter regions. This method has some advantages; however, it requires at least one thermal diffusion operation, complex optimization of the laser operation, and, potentially, additional deposition and annealing steps.
U.S. Pat. No. 8,105,869 issued on Feb. 14, 2012 to B. Gilman discloses the method for forming selective emitters, field-induced emitters, back-surface field regions, and contacts to the functional regions of a solar cell by essentially electrical means and without conventional thermal diffusion and masking processes. The process includes forming conductive layers on both sides of an intermediate solar-cell structure, performing electrical and thermal treatment by passing electrical current independently through the front-side conductive layer and the back-side conductive layer, thus forming the selective emitters, the selective BSF regions, selective emitter contact regions, and contacts to the selective BSF regions. The obtained structure is then subjected to pulse electrical treatment by applying a voltage pulse or pulses between the front and back conductive layers to form the field-induced emitter and the field-induced BSF region. After the conductive layers are removed, a final solar cell is obtained.
Although the above method comprises using a Joule heating and electrical pulse treatment for forming functional regions of the solar cell, it applies essentially to the front-side cells, in which selective emitters, field-induced emitters and contacts to those regions are formed on the front-side of the silicon substrate, while the backside doped regions (selective BSF) and field-induced BSF regions are formed on the back side of the substrate. Also, in this method an electrothermal treatment (Joule heating) is conducted, separately or in parallel, by passing electrical current through the front-side conductive layer and by passing another electrical current through the backside conductive layer.
Bulgarian Patent No. BG109881 issued on Dec. 30, 2008 to Petko Vitanov, et al, describes a solar cell with an emitter, that is made in the form of an inversion layer, (further referred to as a field-induced emitter in the present application) wherein the front-side field-induced emitter is formed by an electric field generated by an electric charge developed in a dielectric antireflective coating on the front surface of the solar cell. However, this type of cell requires formation of selective N+ doped emitters and BSF regions by means of conventional high-temperature diffusion.
Finally U.S. Patent Application Publication No. 2010159633 (published on Jun. 24, 2010; inventors: Lee Boung-Kyu et al) describes method of producing photovoltaic device (i.e.SC) using Joule heating-induced recrystallization method. The described method applies to forming a photo conversion layer (i.e. a photoactive substrate) and not to forming any functional doped regions such as selective emitters and others.
In view of foregoing there is a desire to build a BS-SC wherein all critical functional regions are formed on the back side of the silicon substrate by the single process of Joule heating and following electrical pulse treatment. Such solution will provide a significant reduction of a manufacturing cost.